1. Field of the Invention
The present invention relates to an apparatus for forming a beam member for a torsion beam in a rear wheel suspension system of an automobile.
2. Description of the Related Art
A torsion beam for a rear wheel suspension system of a vehicle comprises a beam member, and trailing arms at both sides of the beam member, and acts to maintain a stable posture of the vehicle through torsion of the beam member against centrifugal force exerted upon rotation of the vehicle. Accordingly, it is necessary for the torsion beam to have strong torsional rigidity and bending rigidity.
Initially, high torsional rigidity and bending rigidity torsion beams were produced in such a manner that, after a steel plate was bent into a beam member having a U-shaped or V-shaped cross section, a reinforcing torsion bar was provided parallel to the beam member inside the beam member, and then welded to the beam member with a reinforcing plate, followed by welding the trailing arms to both sides of the beam member.
However, although such a torsion beam may have sufficient torsion rigidity and bending rigidity via reinforcement of the torsion bar, the number of components for the torsion beam, such as a torsion bar and a reinforcing bar, is increased, and results in increasing the number of assembly processes and welding length, thereby remarkably reducing productivity of the torsion beam while increasing the frequency in producing defective products due to difficulties in the management of welding tolerances. Furthermore, the increased number of components also causes a reduction in fuel efficiency of the vehicle due to the increased weight of the vehicle.
In order to solve the problem of the prior art, a torsion beam, as disclosed in Korean Utility model No. 278494 issued to the applicant of the present invention, was suggested, which is produced by pressing a cylindrical pipe into a beam member having a central portion of a U-shaped or V-shaped two-fold cross section and both sides, each having a rectangular cross-section for securing stable welding to trailing arms, thereby providing sufficient torsional rigidity and bending rigidity required for the torsion beam of the rear wheel suspension system while enhancing the productivity caused by reduction in the number of components, the assembly process, and the length of welding. Furthermore, the torsion beam of Korean Utility model No. 278494 also provides the advantageous effect of reducing the frequency of defective products caused by management of the tolerance upon welding, and of enhancing fuel efficiency of the vehicle by reducing the weight of the vehicle.
However, when forming the beam member having different cross sections at different portions by pressing the pipe, as mentioned above, the number of processes for forming the beam member is not sufficiently reduced. Accordingly, in view of the fact that the increase in the number of processes causes not only reduction in productivity, but also increases manufacturing costs, thereby adversely affecting price competitiveness of the products, it is necessary to form a beam member with a minimum number of processes.
U.S. Pat. No. 6,119,501 discloses a method and device for forming a product having a U-shaped or V-shaped cross section by pressing a cylindrical pipe by use of a top die and two bottom dies having different shapes or by use of a top die and a bottom die comprising several parts movable with respect to one another.
A manufacturing process using two bottom dies comprises a preform forming process, which is performed in a state of The pipe fixedly held on a mandrel, and a secondary final forming process. In the manufacturing process using a bottom die comprising movable parts, the device comprises a shelf shaped top die, and a bottom die consisting of one middle part and two side parts, so that the beam member is formed by virtue of corporation of The top die and the bottom die in which the side parts move along slanted surfaces of the middle part.
It is considered that the method and device of U.S. Pat. No. 6,119,501 is useful for forming products with an entirely simple structure having a U-shaped or V-shaped cross section. However, when forming products having different cross sections at different portions of the product, like the beam member used for the torsion beam for The automotive rear suspension system, it is difficult to form a beam member having complete symmetry about the vertical axis of the beam member, since serious distortion or wrinkling of the pipe could occur at steep changes of the shape during deformation when external force is applied to the outer surface of the pipe through the dies.
As such, if complete symmetry between left and right sides of the beam member is not ensured, not only does deterioration of product quality occur due to non-uniform spring rigidity of the product, but misalignment also occurs between wheels and a vehicle frame due to displacement of the trailing arms welded to both sides of the beam member. Additionally, the torsion beam cannot ensure uniform torsion upon rotation of the vehicle, thereby causing a deterioration in not only the performance of a handle, but also durability of the torsion beam, which is caused by strong torsional eccentricity towards one side of the torsion beam. Accordingly, the method and device of U.S. Pat. No. 6,119,501 cannot be used for forming products having different cross sections at different portions of the product.
Accordingly, as with the beam member used for the torsion beam for the rear wheel suspension system of the automobile, when a product is formed to have different cross sections at different portions of the product in complete symmetry between left and right sides of the product without distortion or winkling, the product must be formed by progressive forming processes using several forming devices. Accordingly, not only do installation costs for the devices become very burdensome, but the productivity is also adversely affected due to an increase in the number of process steps, thereby increasing the manufacturing costs while adversely affecting price competitiveness of the products.